How to Design Efficient Street Lighting-Part-3

(C) Lighting Factor:

(1) Maintenance Factor (Light Loss Factors) (MF)

  • The Maintenance Factor (Light loss factor) is the combination of factors used to denote the reduction of the illumination for a given area after a period of time compared to the initial illumination on the same area.
  • The efficiency of the luminaire is reduced over time. The designer must estimate this reduction to properly estimate the light available at the end of the lamp maintenance life.
  • Luminaire maintenance factors vary according to the intervals between cleaning, the amount of atmospheric pollution and the IP rating of the luminaire.
  • However, it is proposed to consider maintenance factor of not less than 0.5 for LED Road lighting installations for IP66 rated luminaires.
  • The maintenance factor may range from 0.50 to 0.90, with the typical range between 0.65 To 0.75
  • These maintenance factor values shall be adopted for the purposes of producing the lighting simulation design.
  • The maintenance factor is the product of the following factors.
  • LLF = LLD x LDD x EF
  • Mostly We consider Maintenance factor from 0.8 to 0.9
  • We have to choose Maintenance factor carefully by increasing maintenance factor 0.5 the spacing of pole increasing 2 meter to 2.5 meter
Maintenance Factor Max. Spacing of Pole (Meter)
0.95 43
0.9 40.5
0.85 38
0.8 36

(a) Lamp Lumen Depreciation Factor (LLD)

  • As the lamp progresses through its service life, the lumen output of the lamp decreases. This is an inherent characteristic of all lamps. The initial lamp lumen value is adjusted by a lumen depreciation factor to compensate for the anticipated lumen reduction.
  • This assures that a minimum level of illumination will be available at the end of the assumed lamp life, even though lamp lumen depreciation has occurred. This information should be provided by the manufacturer. For design purposes, a LLD factor of 0.9 to 0.78 should be used.

(b)Luminaire Dirt Depreciation Factor (LDD).

  • Dirt on the exterior and interior of the luminaries and to some on the lamp reduces the amount of light reaching the roadway.
  • Various degrees of dirt accumulation may be anticipated depending upon the area in which the luminaire is located. Industry, exhaust of vehicles, especially large diesel trucks, dust, etc, all combine to produce the dirt accumulation on the luminaries.
  • Higher mounting heights, however, reduce the vehicle-related dirt accumulations.

LDD factor of 0.87 to 0.95 should be used. This is based on a moderately dirty environment and three years exposure time.

(c) Equipment Factor (EF).

  • Allows for variations inherent in the manufacture and operation of the equipment (i.e., luminaries, system voltage and voltage drop).
  • It is generally assumed to be 95%.

(2) Coefficient of Utilization (CU):

  • Coefficient of Utilization is the ratio of the luminous flux from a luminaire received on the surface of the roadway to the lumens emitted by the luminaire’s lamps alone.
  • Coefficient of Utilization should be maximum.
  • Coefficient of Utilization differs with each luminaire type, and depends upon mounting height, road width, and overhang.
  • The coefficient of utilization (K) should be over 30% or the utilance above 40% for the road, highway, square or enclosure. Luminaires or floodlights should not by placed far from the area to be lit or, where appropriate, light projection beyond the useful zone should be minimized (K = average maintained illuminance multiplied by the surface calculation and divided by the lumens installed).

Various Factors

Type Luminaries Dirt Depreciation Luminaire Lumen Depreciation Total Light Loss Factor
LED 0.9 0.85 0.765
HPS 0.9 0.9 0.81
LPS 0.9 0.85 (0.7 for 180W) 0.765 (0.63 for 180W)

Light Loss Factors

Type of Lamp Laminar Dirt description Light Loss Factor
HPS 0.88 0.74
Induction 0.88 0.62
LED 0.88 0.72

Maintenance factors

Cleaning intervals (months) Pollution category
High Medium Low
12 0.53 0.62 0.82
18 0.48 0.58 0.8
24 0.45 0.56 0.79
36 0.42 0.53 0.78
Maintenance Factors for 36 month cleaning interval
Factors IP5X IP6X
Pollution category Pollution category
Low Medium High Low Medium High
LMF 0.88 0.82 0.76 0.9 0.87 0.83
LLMF 0.89 0.89 0.89 0.89 0.89 0.89
MF 0.78 0.73 0.68 0.80 0.77 0.74


(E) Lighting Uniformities

(1) Lighting Uniformities

  • Uniformity is a description of the smoothness of the lighting pattern or the degree of the intensity of bright and dark areas on the road.
  • Uniformity is a measure of how evenly distributed the light on the road is, which can be expressed as Overall Uniformity (UO) and Longitudinal Uniformity (UL).
  • The uniformity ratio shall not exceed 4:1 and preferably should not exceed 3:1 except on residential streets, where 6:1 may be acceptable.

(a) Overall uniformity:

  • In design, the overall uniformity (UO) is expressed as a ratio of the minimum to the average luminance on the road surface of the carriageway within the calculation area.
  • UO=Lmin / Lave

  • It is a measure of how evenly or uniformly illuminate on the road surface.
  • A good overall uniformity ensures that all spots and objects on the road are sufficiently lit and visible to the motorist.
  • The industry accepted value for UO is 30 to 0.40.

(b) Longitudinal uniformity:

  • The longitudinal uniformity (UL) is expressed as the ratio of the minimum to maximum luminance along the center line of a lane within the calculation area.
  • UL=Lmin / Lmax.
  • Longitudinal uniformity is a measure to reduce bright and dark bands of light appearing on road lit surfaces. The effect can be somewhat hypnotic and present confusing luminance patterns.

  • It is a measure to reduce the intensity of bright and dark banding on road lit surface.
  • A good level of longitudinal uniformity ensures comfortable driving conditions by reducing the Pattern of high and low luminance levels on a road (i.e. zebra effect).
  • It is applicable to long continuous roads.

 Combination of Overall Uniformity and Longitudinal Uniformity:

  • The picture on the left shows a road with good UO while the picture on the right has low level of UO. The Road is more visible in the road with higher UO. Having higher UO allows the motorist to see the road clearly and anticipate potential road hazards (e.g. open manholes, road excavations, sharp objects on the road, people crossing the street).
  • The picture on the right shows a road with low level of UL demonstrating the ‘Zebra Effect’ while the picture on the left has high level of UL without ‘Zebra Effect’.
  • The ‘zebra effect’ can cause discomfort to motorists, posing a risk to road safety. Ensuring good level of uniformity can reduce the luminance level needed.

Lighting Levels

Category Eave ( LUX) Emin  LUX) Uniformity ratios
Emax : Emin Eave : Emin
Express & Main street 30 15 3:01 2.5:1
Suburban shopping street 25 10 5:01 3:01
Subsidiary street 15 10 5:01 3:01
Other streets 15 5 10:01 5:01


 Lux Level

Road Classification Area Classification Average  Lux Uniformity Ratio (Aver./Min.)
Arterial (Minor & Major) Commercial 12 3 to 1
Intermediate 9
Residential 6
Collector (Minor & Major) Commercial 8
Intermediate 6 4 to 1
Residential 4
Local Commercial 6
Intermediate 5 6 to 1
Residential 3
Alleys Commercial 4
Intermediate 3 6 to 1
Residential 2
Sidewalks (Roadside) Commercial 3 3 to 1
Intermediate 6 4 to 1
Residential 2 6 to 1
Pedestrian Ways 15 3 to 1


Illumination for Intersections

Functional Classification Average Maintained Illumination at Pavement by Pedestrian Area Classification in Lumen Uniformity
High Medium Low Eavg/Emin
Major/Major 37 28 19 32
Major/Collector 31 24 16 32
Major/Local 28 22 14 32
Collector/Collector 26 19 16 43
Collector/Local 23 17 11 43
Local/Local 19 15 9 65


Illumination for Pedestrian Areas

Maintained Illuminance Values for Walkways
Area Classification Description E avg (Lux) EV min (Lux) E avg/Emin
High Pedestrian Conflict Mixed Vehicle and Pedestrian 22 11 43
Areas Pedestrian Only 11 5 43
Medium Pedestrian Pedestrian Areas 5 2 43
Conflict Areas
Low Pedestrian Rural/Semi-Rural Areas 2 1 108
Conflict Areas Low Density Residential (2 or fewer dwelling units per acre) 3 1 65
Medium Density Residential (2.1 to 6.0 dwelling units per acre) 4 1 43
Pedestrian Portion of Pedestrian/Vehicular Underpasses Day 108 54 43
Night 43 22 32

 (2) Surround Ratio (SR):

  • Road lighting should be illuminate not only the road, but also the adjacent areas so motorists can see objects in the periphery and anticipate potential road obstructions (e.g., a pedestrian about to step onto the road).
  • The SR is the visibility of the road’s periphery relative to that of the main road itself.
  • As per industry standards, SR should be at least 50.
  • Figure show how road lighting should illuminate both the main road and its periphery.

About Jignesh.Parmar (B.E,Mtech,MIE,FIE,CEng)
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

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